The present invention relates generally to a method and apparatus for the injection of gas and plastic into a mold cavity. More particularly, the present invention relates to a method and apparatus for plastic injection molding in which a predetermined quantity of a pressurized inert gas is introduced into a quantity of plastic melt while the melt is flowing from the barrel into the mold cavity.
Injection molds typically comprise stationary and moving mold halves (i.e., the core side and the cavity side) which are closed and clamped together to form a mold cavity therebetween for shaping articles from thermoplastic compositions. The thermoplastic is heated into a molten condition and is injected under pressure through a nozzle and into the mold cavity by means of a screw ram. Injection pressures of 2,000 to 10,000 psi are common at the gate locations. The plastic is allowed to cool to sufficiently harden the thermoplastic whereupon the mold is opened and the hardened articles are removed.
A traditional plastic molding problem is the formation of surface distortions or xe2x80x9csink marksxe2x80x9d on the appearance side of the molded article opposite to ribs or bosses on the backside of the article and resulting from the high injection pressures used to fill the cavity, the pack out pressure, and/or from an uneven pressure gradient resulting from the injection pressure at the gate being higher than the pressures at the extreme ends of the molded article. High injection pressure can cause strain marks or molded-in strain in the hardened article, resulting in warpage at once, or over a period of time after molding, or if the end use of the molding is in a high temperature area. When ribs are formed in the moldings, the wall thickness versus rib configuration can cause the ribs to buckle or bend the molding, due to shrinkage differential. In large projected area moldings where the plastic cannot flow from the gate to the end of the molding, hot runner molds are needed and high clamping forces (e.g., 1,000 to 10,000 pounds per square inch of projected area) are required to hold the mold halves together. These molds are costlier to build, and the gates from the hot runners can add weld lines to the molded product. Injection molding machines which can provide these high clamping forces are costly to operate.
The molding of thick-sectioned parts presents its own demands. One of the approaches taken in the molding of such parts is the use of structural foam. A part produced according to this process is relatively light in weight. However, the surface finish of such a product is poor and typically requires extensive finishing operations. Also, the method required for molding structural foam parts is cumbersome, since it necessitates relatively long cycle times (for the cooling of the plastic in the mold). The requisite method also produces parts having inconsistent surface finishes due to streaking. As the flowing plastic material enters the mold cavity, bubbles produced by the foaming agent can form at the front of the flow near the point of entrance. Streaking results as the flowing material passes by the bubbles.
Another approach taken in the molding of thick-sectioned parts is a process that has come to be known as xe2x80x9cgas assisted injection moldingxe2x80x9d in which an inert gas is injected through the plastic injection nozzle and directly into the thick areas of the melted thermoplastic, thereby creating hollow sections in the part. According to known gas assisted injection molding methods, the gas is injected after the molten plastic resin has substantially filled the mold cavity. With such conventional gas assisted molding process, sink marks and warpage can be minimized and possibly be eliminated. The gas forms hollow portions in the body of the material and/or hollow channels (gas channels) in the thicker portions, such as between the surface of the part and a backside detail, such as a rib. For ribbed products, the base of the ribs must be made thicker or wider in order to help direct the gas channel, which is just the opposite of normal design practice with plastic where ribs are made as thin as possible to try to eliminate shrinkage and shorten cooling and cycle times. With the gas channel at the base of a rib, material will shrink away from the inside surface of the channel as the molded part cools because the material is hottest at the center of the section. Therefore, as the plastic part shrinks during cooling, the sink marks on the visible outside surface of the parts can be minimized.
A disadvantage in conventional gas assisted molding technology is that the possibility of achieving Class A surfaces on the appearance surfaces of the molded parts is inhibited by shadow marks caused by gas holes in the thicker areas of the molded articles, and gas permeation caused by the gas not being retained in the thicker areas and overflowing into the wall thickness of the articles. This often causes thinning and weakening of the wall, raised areas, and blush marks.
Injection molding of parts utilizing a pressurized gas source is shown, for example, in U.S. Pat. No. 5,344,596, issued on Sep. 6, 1994, to Hendry for METHOD FOR FLUID COMPRESSION OF INJECTION MOLDED PLASTIC MATERIAL. While the method of this patent represents an improvement in the molding of articles of the type shown therein through the use of a gas, there remains a need for improvements in forming low cost articles.
Accordingly, it is an object of the present invention to provide an improvement to the art of gas assisted plastic injection technology. An additional object of the present invention is to provide a gas assisted molding method and apparatus that reduces the overall cost of gas delivery systems normally associated with gas assisted injection molding.
A still further object of the present invention is to provide such a method and apparatus which demonstrates reduced costs through, for example, electric power consumption. Yet another object of the present invention is to provide such a method and apparatus which demonstrates reduced costs through, by way of a further example, eliminating costly gas units.
A further object of the present invention is to provide such a method and apparatus which demonstrates reduced costs through, by way of yet an additional example, eliminating sophisticated and costly electrical conduits. Still a further object of the present invention is to provide such a method and apparatus which eliminates the need for a high pressure gas compressor and its associated maintenance problems.
Still an additional object of the present invention is to provide such a method and apparatus which eliminates the need for spillover of molten material into a spillover cavity. Yet a further object of the present invention is to provide such a method and apparatus which eliminates shadow, permeation, and hesitation marks.
An additional object of the present invention is to provide such a method and apparatus which relies upon the same power source to inject both plastic and gas. A further object of the present invention is to provide such a method and apparatus which eliminates clogged gas injection pins by eliminating the need for the pin itself in the mold cavity.
Yet an additional object of the present invention is to provide such a method and apparatus which allows for the use of lower cost pins outside of the mold cavity in the plastic flow. An additional object of the present invention is to provide such a method and apparatus which allows the steps of the process to be controlled by linear distancing, thus eliminating the need for a controlling timer.
Still a further object of the present invention is to provide such a method and apparatus which establishes a correct volume and pressure of gas prior to the step of plastic injection. Still a further object of the present invention s to provide such a method and apparatus which allows relatively easy yet accurate control of gas volume as well the gas pressure.
Yet a further object of the present invention is to provide such a method and apparatus which allows for the relatively easy change of both gas volume and pressure as required from task to task. An additional object of the present invention is to provide such a method and apparatus which relies upon a low-cost, low-maintenance, low pressure unit. A further object of the present invention is to provide such a method and apparatus which allows for the introduction of gas into the melt flow at a controlled rate as opposed to a single shot, thus creating a virtually continuous gas flow.
These and other objects of the present invention are achieved by the provision of a gas assisted molding apparatus having gas introduced into the molten plastic charge as the charge is flowing into the mold cavity. The gas and plastic are simultaneously injected such as in the barrel or at the nozzle end of the injection molding machine. Optionally, the gas may be injected into a hot runner manifold or into the cavity itself simultaneously with the plastic. The apparatus includes a conventional gas injection mold, a source of inert gas (such as nitrogen), and an injection assembly. A common power source is used for both the injection of the plastic and the gas. This insures that the gas and plastic will flow together into the mold in a simultaneous manner and at the same pressure. (Conversely, using separate power sources, one for plastic and the other for the gas, will result in one fluid overcoming the other, thus providing an unacceptable product.)
The gas is introduced into the plastic at any point of the process while the plastic is being injected. The volume of plastic is pressure-dependent upon the desired volume and pressure of the gas. According to the method, the screw in the injection barrel is rotated to deposit a preferred quantity of plastic (less the anticipated gas volume) in the front of the barrel. The molten plastic is forced into the mold by moving the screw longitudinally forward in the barrel by the hydraulic system normally incorporated into the injection molding machine. During the movement of the screw at any time during the injection stroke, a measured amount of gas is injected into the plastic melt through an injection nozzle (or pin) into the flowing molten plastic material using the same hydraulic pressure used to move the screw to inject the plastic out of the barrel and into the mold cavity. The injection of gas into the plastic is accomplished by operating a hydraulic cylinder to compress the gas in a gas chamber until it reaches a value equal to the pressure of the plastic located in the front of the barrel. The power to activate the hydraulic cylinder is flow-coupled to the hydraulic pressure used to force the plastic out of the end of the barrel. In this situation, the gas pressure at the pin equals the pressure moving the plastic by the screw into the mold, thus resulting in the simultaneous injection of plastic and gas into the mold to create a hollow article. The components of the system are then reset to their predetermined positions in preparation of the next molding cycle.
In alternate embodiments, the plastic could be injected by an electric or pneumatic operated mechanism and the gas could be injected into the molten plastic materials by the same power source and at the same pressure.